The invention relates to a method of detaching a segment disposed on a carrier from a material layer extending in a layer plane and having a specific layer thickness, by means of a laser pulse passing through the carrier.
A method of this type known from DE 196 40 594 provides for the layer disposed directly on the carrier being destroyed as such by the effect of light and detachment of the segment therefore occurring as a result of the destruction.
The destruction of part of the material layer or of a layer specially provided for the purpose application takes place with a relatively long time scale of the order of nanoseconds with the result that in the end the entire layer is heated up and also that the segment is detached with an insufficiently precise boundary surface.
The so-called LIFT processes work in a similar fashion, producing even large quantities of molten material, which give rise to material contamination around the region to be detached and are especially undesirable in microtechnology.
The objective of the invention is therefore to detach segments from a material layer with as little thermal stress and as few thermal side effects as possible.
In a method of the type described in the introduction, this objective is achieved according to the invention in that the laser pulse within a partial layer volume of the segmentxe2x80x94the said partial layer volume butting against the carrier, lying in the plane of the layer within an extent of the beam cross-section of the laser pulse and extending transversely to the layer plane via a part of the layer thicknessxe2x80x94produces superheated matter in a state of thermodynamic non-equilibrium and of a density similar to the solid state and in that a cohesive, solid partial layer remains in the segment on the side of the partial layer volume opposite to the carrier, the said partial layer being urged away from the carrier by the superheated matter.
The advantage of the solution according to the invention is observable in that, as a result of the specially produced superheated matter in a state of thermodynamic non-equilibrium, instead of the conventional evaporation process of the material, a so-called explosive evaporation now occurs, which on the one hand produces a very high pressure consequently causing marked acceleration during detachment of the cohesive, solid partial layer which, for example, permits precise detachment of the segment from the layer and which on the other hand, on account of the short time duration, avoids thermal stressing of the cohesive, solid partial layer forming the segment. The result of this is that, with the method according to the invention, on the one hand large forces of acceleration are available for the cohesive, solid partial layer, the acceleration forces furthermore being combined with the fact that, on account of the explosive evaporation, the cohesive, solid partial layer undergoes far less thermal stress than with the known method, with the result that here the adverse thermal secondary effects are substantially eliminated.
In particular in the solution according to the invention, a highly excited non-thermodynamic state is achieved with ultra-short laser pulses, in which arrangement the electron temperature may far exceed that of the phonones. The stored energy is transferred from the electrons to the phonone system with a characteristic material-dependent time, for example of the order of magnitude between 50 femtoseconds and 2 picoseconds, in a volume which is determined by the extent of the partial layer volume in the plane of the layer and the extent of the partial layer volume transversely to it, due to the thermal penetration depth of the electrons which, for example at a pulse duration of 100 femtoseconds, is of the order of magnitude of 50 nm. Thus the phonone system can be heated extremely rapidly to a zone above the critical temperature without a conventional evaporation process taking place.
In particular, the high pressure briefly arising due to the explosive evaporation brings about the already described greater acceleration of the cohesive, solid partial layer, which is also responsible for the fact that the thermal stressing of the material is less in the cohesive, solid partial layer.
In an embodiment of the method according to the invention, superheated matter is preferably in the state of thermodynamic non-equilibrium at a temperature above the critical temperature.
Advantageously, the superheated matter according to the invention is producible only with laser pulses, the pulse duration of which is less than 100 picoseconds and the pulse duration of which is so short that it impossible to establish a thermodynamic equilibrium.
In the solution according to the invention, it is especially advantageous if, at least at the beginning, the material of the material layer is present in the superheated matter substantially unchanged and the matter thus simply possesses more energy than before the action of the laser pulse but does not itself change for example chemically.
In the method according to the invention, it is furthermore advantageous if the superheated matter from material of the material layer expands substantially stoichiometrically during detachment of the segment, i.e. that the material composition does not change during expansion of the superheated matter and thus no contaminants occur due to degrading material, with the result that in particular the arising cohesive, solid partial layer, which is detached from the carrier, is not contaminated with constituents resulting from degradation of the superheated matter and thus from non-stoichiometric expansion.
In an embodiment according to the invention, it is furthermore advantageous if the storage of energy in the superheated matter takes place in the electron system only until thermal losses occur.
As regards the description of the method according to the invention, so far it has merely been assumed that at least one laser pulse is necessary. It is especially advantageous if detachment of the segment is performed with a single laser pulse.
Especially great accelerations of the cohesive, solid partial layer can then be achieved if the cohesive, solid partial layer is accelerated away from the carrier by hydrodynamic expansion of the superheated matter in the partial layer volume. An extremely precise tearing of the cohesive, solid partial layer from the surrounding layer is hereby achievable, especially when the segment is still connected with the surrounding layer.
In a large number of applications it is sufficient to detach, i.e. for example to lift off, from the carrier only the cohesive, solid partial layer as a segment from the layer.
However, it is especially advantageous if the cohesive, solid partial layer is accelerated in the direction of a substrate. Here in particular the solution according to the invention lends itself as especially suitable since, by comparison with the state of the art, it is capable of exposing the cohesive, solid partial layer to large forces of acceleration and thus of moving it on to a substrate with great precision.
In this solution it is especially advantageous if the cohesive, solid partial layer is fixed on the substrate, it being possible for this purpose to prepare the substrate for receiving the cohesive, solid partial layer in such a way that adhesion contributes to the fixing.
However, it is especially advantageous if the energy of impact of the cohesive, solid partial layer on the substrate leads to adequate fixing on the latter. Here too, in particular the solution according to the invention can be used highly advantageously since, on account of the large forces of acceleration, it is predestined for fixing the cohesive, solid partial layer on the substrate by means of its impact energy.
The fixing takes place for example in the form of a so-called xe2x80x9ccold bondingxe2x80x9d of the cohesive, solid partial layer with the substrate.
In this arrangement the substrate is advantageously positioned at a distance from the carrier at which impact-determined fixing of the cohesive, solid partial layer on the substrate occurs.
It is also especially advantageous if the size of the beam cross-section is selected so that the impact energy leads to adequate fixing on the substrate.
It is also similarly advantageous if the energy of the laser pulse is selected so that the impact energy leads to adequate fixing on the substrate.
Another advantageous solution provides for the pulse duration of the laser pulse being selected so that the impact energy leads to adequate fixing on the substrate.
However, as an alternative to deposition of the cohesive, solid partial layer it is also similarly conceivable for the cohesive, solid partial layer to be detached by detaching means at the time of its detachment from the carrier, with the result that no movement of the cohesive, solid partial layer from the carrier on to the substrate is necessary over considerable distances.
As regards the pulse duration of the laser pulse, no detailed information has so far been given. It is therefore especially advantageous if the pulse duration of the laser pulse is determined in such a way that the partial layer volume extends transversely to the plane of the layer over a thermal penetration depth of electrons heated during the pulse duration of the laser pulse.
An especially advantageous solution provides for the pulse duration of the laser pulse being smaller than the material-dependent thermal relaxation time of electrons of the material, with the result that an extremely rapid and efficient heating of the matter in the partial layer volume, and therefore minimal thermal stressing of the cohesive, solid partial layer to be detached, are possible.
In this arrangement, according to the invention the pulse duration may be constant as observed over the beam cross section may be constant or it may vary over the beam cross section.
Especially advantageous pulse durations are shorter than 50 picoseconds, even better shorter than 20 picoseconds.
As regards the materials to be used for the material layer in the method according to the invention, an extremely wide range of solutions is conceivable.
One advantageous solution provides for the material layer being selected so that the laser pulse couples to free electrons of the material.
Another solution provides for the material layer being selected so that the laser pulse itself produces the electrons necessary for coupling by means of multiple photon absorption in the material layer.
A further solution provides for the material layer being selected so that the laser pulse itself produces the electrons necessary for coupling by means of an avalanche breakdown in the material layer.
In respect of the distribution of the energy density in the laser pulse, no detailed information has been given in connection with the above description of the individual embodiments. Thus, for example, the energy density in the beam cross-section could exhibit a Gaussian distribution.
However, in connection with the solution according to the invention it is especially advantageous if the local energy density within a core region of the beam cross-section exhibits values of the same order of magnitude, i.e. that the local energy density within the beam cross-section varies less with a Gaussian distribution.
It is especially advantageous if the energy density values within the core region of the beam cross-section show a substantially flat course.
A profile such as this is for example described as a so-called hat-top profile.
In such a course of the energy density within the core region, the energy density outside the core region is preferably substantially lower, but similarly substantially of equal magnitude.
An especially advantageous solution provides for the values of the energy density of the laser pulse in a marginal region outside the core region of the beam cross-section being higher than the values within the core region. With this there is the possibility of achieving a substantially constant energy uptake of matter within the partial layer volume corresponding to the core region, and of designing the energy uptake in a partial layer volume marginal region corresponding to the marginal region of the beam cross-section to be even more strong, which is advantageous especially when the segment is to be detached from the surrounding layer since in this way the detachment can be improved and rendered precise along a tear line around the segment.
In this arrangement it would also suffice to increase the energy density at individual points outside the core region. It is especially advantageous, however, if the values of the energy density in the marginal region around the whole core region are elevated by comparison with the core region, so that the core region is surrounded by a substantially closed marginal region of elevated energy density and thus the detachment of the segment from the layer can be achieved especially advantageously.
As an alternative to detachment of the segment from the surrounding layer, within the framework of the solution according to the invention in the variants previously described it is also conceivable to pre-structure the segment. This means that the layer thickness of the material layer around the segment is already reduced before impingement of the laser pulse. In this arrangement the reduction may be over the whole layer thickness, but it may also only extend over a part of the layer thickness. In all cases pre-structuring has the advantage that the external contour of the resulting segment can be defined even more precisely and that even after detachment of the cohesive, solid partial layer, the external contour exhibits the desired, precise shape whereas, without pre-structuring of the external contour, there is always uncertainty because, for example due to energy-density inhomogeneities in the laser beam or inhomogeneities in the material layer itself, the external contour of the segment deviates so far from the desired shape that this is undesirable and for example reduces the possible applications of the segment or renders its use impossible in a particular case.
Moreover, an especially advantageous embodiment of the method according to the invention provides for the partial layer being permanently deformed by a defined variation of the individual regions of the partial layer volume by energy supplied by the laser pulse.
In this arrangement, the energy supplied by the laser pulse can be varied either in that the energy density of the laser pulse varies over the cross-section of the beam and thus variably large quantities of energy are supplied to individual regions of the partial layer volume, or also through time variation of the action of the laser pulse on the individual regions of the partial layer volume, with the result that a variation of the quantities of energy supplied to these individual regions is also possible by this means.
Here, deformation of the partial layer must be understood as a shaping of the type which results in a deviation from the shape of the partial layer prior to the action of the laser pulse on the carrier.
This kind of deformation of the partial layer can now be utilised advantageously in various ways.
For example, an advantageous solution provides for the partial layer being deposited on the substrate as a permanently formed component.
This is advantageous for example in cases when the partial layer is to be laid out fully on an uneven surface, with the result that, due to a deformation corresponding to the shape of the uneven surface, an improved full laying out or even fixing is achievable.
This arrangement preferably provides for this permanently formed component being used on the substrate as a functional structural component.
A very wide range of possible uses are conceivable as functional parts of this type.
For example, a permanently formed component of this type may be used variously as an electrical functional member, for example as a ladder bridge, as part of a capacitor, as part of a spring contact, or as part of a switch in micro switches, especially in micro-system technology.
Alternatively or in addition thereto, a permanently formed component of this type may however also be used as a mechanical part, for example as a spacer or spring in micro-system technology.
Especially advantageously, the method according to the invention can be utilised whenxe2x80x94as is the case in particular with functional partsxe2x80x94the partial layer is fixed to the substrate only with a partial region and the rest of the partial region therefore stands away from the substrate and so various functions can be fulfilled by means of this separated partial region.
In addition, the objective cited in the introduction is also achieved according to the invention by a device for detaching a segment from a material layer extending in a plane of the layer and having a specific layer thickness, the said segment being disposed on the carrier, the said device comprising a holder for positioning the carrier, a laser system for producing individual laser pulses and a beam-guide lens system, which allows a laser pulse to pass through the carrier for the purpose of detaching the segment, in that the laser pulse within a partial layer volume of the segmentxe2x80x94the said partial layer volume butting against the carrier, lying in the plane of the layer within an extent of the beam cross-section of the laser pulse and extending transversely to the layer plane via a part of the layer thicknessxe2x80x94produces superheated matter of a density similar to the solid state, in a state of thermodynamic non-equilibrium and in particular at a temperature above the critical temperature, and in that a cohesive, solid partial layer remains in the segment on the side of the partial layer volume opposite to the carrier, the said cohesive, solid partial layer being accelerable by the superheated matter.
Further advantageous embodiments of the device according to the invention arise in correspondence with the advantageous embodiments of the method according to the invention which are described in detail.
The method and devices according to the invention are particularly suitable for manufacturing miniaturised structural assemblies in which the manufacture of structured surfaces is necessary. Thin metallic layers, semiconducting layers, superconducting layers, ceramic layers and insulating layers or layers of organic substances with depressions, troughs or ridges within the micrometric range often have to be manufactured for this.
The applications in semiconductor technology are found, for example, in the secondary processing of mass-produced goods. One example of use is the flexible circuitry of electrode matrices (free programmable gate arrays), or the balancing of parameters such as for example the balancing of resistances and oscillation frequencies. Other possible uses are found in the repair of complex switching circuits or of lithographic masks, in the client-specific manufacture and modification of structural members such as e.g. ASICS, and in the prototype development of structural members.
A further application is found quite generally in the detachment of structural components or structural assemblies from carrier materials following their isolation. Applications for die bonding and flip-chip bonding are of especial interest here.
An especially advantageous variant of the device according to the invention provides for different quantities of energy being suppliable with the laser pulse to the different regions of the partial layer volume, the partial layer being permanently deformable by the different quantities of energy.
Here it is especially advantageous if the partial layer is placeable on a substrate as a component and thus preferably represents a functional member.
Further features and advantages of the invention are the subject of the following description and of the graphic representation of a few embodiments of the solution according to the invention.